[3.0] Valgrind detected TextureSet leak and invalid access

Memory leak:

If a public API TextureSet gets deleted, it sends a
message to update-manager (RemoveTextureSet).
The scene graph side TextureSet gets removed from
update manager, but it never gets deleted.

Invalid memory access:

Currently when a scene graph TextureSet gets deleted
the renderers using that TextureSet are not notified.

So when a Render gets deleted it would call
TextureSet->RemoveObserver( this );

on an already deleted TextureSet.

DALi however doesn't crash at this point,
because the TextureSet is stored in a memory
 pool so the object is still reachable.

Change-Id: Icf0f5a3e55d3ba7537f40db08accad04ae4440f7